This invention relates to an alignment method and an exposure apparatus using the same, usable in the manufacture of semiconductor devices, for example, for relatively aligning a fine electronic circuit pattern, such as ICs, LSIs or VLSIs, formed on the surface of a reticle (first object), with a wafer (second object). More specifically, the invention is concerned with an alignment method and an exposure apparatus particularly effectively arranged to perform an alignment operation in a situation wherein a WIS (Wafer Induced Shift), which is a wafer process error, occurs easily.
Projection exposure apparatuses for semiconductor manufacture should have a performance for projecting a circuit pattern of a reticle onto a wafer at a higher resolution, to meet further increases in the density of an integrated circuit. The projection resolving power for a circuit pattern can be increased by enlarging the numerical aperture (NA) of a projection lens while keeping the wavelength of exposure light fixed, or by shifting the exposure light to a shorter wavelength, e.g., from g-line to i-line or from i-line to an excimer laser, for example.
On the other hand, with further miniaturization of a circuit pattern, the requirement for high-precision alignment between a reticle, having an electronic circuit pattern formed thereon, and a wafer has become more strict.
In the reticle-to-wafer alignment, there are cases using exposure light with which a resist applied to the wafer surface is sensitive and cases using non-exposure light (e.g., 633 nm wavelength from a Hexe2x80x94Ne laser) not sensitizing the wafer resist. Currently, in most cases, non-exposure light is used in practice. This is because of the advantage that non-exposure light is less influenced by semiconductor manufacturing processes. Particularly, since the transmission factor of the resist is high, wafers can be observed independently of the resist characteristic.
The assignee of the subject application has proposed alignment systems using non-exposure light, such as in Japanese Patent Application Laid-Open No. 32303/1988 and Japanese Patent Application Laid-Open No. 130908/1990.
The alignment technique such as above is called a non-exposure light TTL off-axis system, and in such a system, chromatic aberration to be caused as non-exposure light passes a projection optical system, for projecting a reticle pattern onto a wafer, is corrected by an alignment optical system.
In most alignment methods currently used in practice, an optical image of an alignment mark on a wafer is imaged upon an image pickup device such as a CCD camera, and an electrical signal obtainable thereby is image-processed, whereby the position of the wafer is detected.
Currently, there are exposure apparatuses of a type called a stepper, and exposure apparatuses of a type called a scanner. The following description will be made of a stepper type exposure apparatus, as a representative example.
The non-exposure light TTL off-axis method based on image processing can be used in an i-line stepper, but it cannot be used in an excimer stepper using an excimer laser as a light source. This is because a projection optical system when used with light having a wavlength of 633 nm which is an emission wavelength of a Hexe2x80x94Ne laser, for example, produces a very large chromatic aberration, and the correction thereof through an alignment optical system cannot be done with a high NA (numerical aperture).
For this reason, most excimer steppers use a detection system of the non-exposure light off-axis type based on an image processing method wherein, as compared with the conventional non-exposure light off-axis system, a separate off-axis microscope is provided so that observation can be made with non-exposure light, without intervention of a projection optical system.
The non-exposure light off-axis system is a non-TTL off-axis system, rather than a TTL (Through The Lens) system, wherein a projection optical system is not passed through. Thus, any change in the distance between the off-axis microscope and the projection lens, that is, baseline, is a factor of precision deterioration.
In order to suppress the change in baseline to attain high-precision alignment, it is necessary to use in the non-exposure light off-axis system those components which are less thermally influenced or to perform baseline correction frequently.
The non-exposure light TTL off-axis system in an excimer stepper may use a method other than the image processing method. An example other than the image processing method is a dark-field detection method wherein illumination light is not detected and only limited diffraction light is used. A method called xe2x80x9cheterodyne detectionxe2x80x9d may apply.
In the non-exposure light TTL off-axis method wherein limited diffraction light is detected, since the baseline is short, inconveniences involved in the non-TTL non-exposure light off-axis system may be avoided. However, because of the dark-field detection, a problem may arise in the detection rate as compared with a bright-field detection.
In current device production, an appropriate one of the image processing methods and other detection methods described above is selected and used, in consideration of their advantages and disadvantages peculiar to them, to meet the required alignment precision.
However, in order to meet a recent requirement for further improvement in alignment precision, there still remain some problems in relation to a semiconductor process error, which cannot be solved by any of the above-described methods.
A large problem is that there is no measure to compensate for a phenomenon that the shape of an alignment mark becomes asymmetrical due to a process or processes.
An example is a flattening process such as in a metal CMP process or the like. In the CMP process, the structure of an alignment mark may become asymmetrical, which may cause in a global alignment procedure a rotational error (FIG. 1A) or a magnification error (FIG. 1B). This results in a serious problem of decreased precision.
The distortion in the structure of a wafer alignment due to the flattening process may produce a larger error in the dark-field detection system, causing a precision decrease. Therefore, although the stability of the baseline can be accomplished by non-exposure light TTL off-axis system different from one based on the image processing method, because of its higher process sensitivity, it is used in only a few cases in practice.
In order to meet the requirement of miniaturization of ICs, it is important to improve the total overlay precision, including the alignment precision. Recently, particular note has been applied to distortion of a projection optical system when a reticle pattern is transferred to a wafer.
What is to be considered in this respect is variation of distortion due to coma aberration, depending on an illumination mode used or a pattern shape used. Conventionally, the distortion has been defined in relation to the location upon an image plane where a chief ray of light is incident, such that it has been treated as being aberration independent from the NA. However, the asymmetry of an image in practice differs with the NA, due to the influence of non-uniformness of an illumination system or coma of a projection optical system. Since the chief ray does not shift even if the NA is changed, the distortion would not change in accordance with the conventional definition. Nevertheless, an image shifts in practice. This phenomenon, if it occurs, will cause deterioration of the total overlay precision.
The distortion to be treated in the present invention is distortion in a practical sense, also taking into account the asymmetry being variable with the NA, and thus, the definition differs from the conventional one wherein distortion depends on the location on an image plane where a chief ray of light is incident. In this specification, the term xe2x80x9cdistortionxe2x80x9d to be referred to below means the formed distortion wherein the asymmetry variable with the NA is also taken into account. If the word xe2x80x9cdistortionxe2x80x9d in the conventional sense will be used somewhere below, it will be referred to as xe2x80x9cdistortion in the conventional sensexe2x80x9d.
The distortion wherein the symmetry variable with the NA is also taken into account, may be influenced not only by non-uniformness of an illumination system and coma aberration of a projection optical system, but also by the shape of a pattern formed on a wafer. This is because of the reason that, since the intensity distribution of diffraction light varies with the pattern shape, the influence of coma aberration of non-uniformness of the illumination system to the diffraction light also varies with it.
Currently, registration or overlay precision system are used to measure the distortion. Most of such systems use a measurement principle based on image processing. A pattern to be measured has a large size of about 10-20 microns square. An optical image of such a measurement pattern is formed on a photoelectric converting device such as a CCD camera, and the position of the pattern is detected through various image processing procedures, whereby the distortion is detected.
The distortion to be measured is the distortion wherein the asymmetry variable with the NA is also taken into account, as described above. On the other hand, a pattern with respect to which an overlay precision is required in practice in semiconductor manufacture is a pattern of the submicron order. Therefore, if the influence of the pattern shape is taken into account, determining the distortion with respect to a pattern of about 10 microns square currently used in overlay inspection systems is not effective to determine a value which is required in practice.
Some proposals have been made in an attempt to detect practical distortion.
An example is that three lines (lines-and-spaces) of 0.5 micron are provided outside a pattern of 10 microns square. Although a single line of 0.5 micron cannot be detected separately by use of an optical overlay inspection system, if three lines are combined, a positional error can be detected. Another proposal is that, while changing the exposure amount, measurement is made on the basis of a distinction, between resolved or not resolved, of a single line of 0.5 micron.
Since, however, the distortion wherein the asymmetry variable with the NA is also taken into account depends on the pattern shape, only the measurement of three lines (lines-and-spaces) of 0.5 micron does not represent a value of distortion. Particularly, there may occur a large difference with the characteristic or the like of distortion of 0.35 micron square such as a contact hole, such that the above-described method is not sufficient.
There is another problem that a fine pattern of a linewidth of 0.35 micron or less cannot be resolved on the basis of optical measurement methods currently available.
It is accordingly an object of the present invention to provide an alignment method and/or an exposure apparatus by which higher-precision alignment can be accomplished.
It is another object of the present invention to provide an offset measurement system suitably usable in such an alignment method and/or an exposure apparatus.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.